Conventionally, there has been known a gas sensor element using lead-phthalocyanine or an oxide semiconductor such as WO3 or SnO2, that is, a gas sensor element capable of changing an electrical characteristic (for example, a sensor resistance value) with a variation in the concentration of a specific gas, for example, an oxidizing gas such as NOx in an environment or a reducing gas such as CO or HC and detecting the variation in the concentration of the specific gas based on the change of the electrical characteristic. Moreover, there has also been known a gas detecting device using the gas sensor element. Furthermore, there have been known various control systems using the gas detecting device, for example, a system for controlling the switching operation of a flap for switching the introduction of outside or inside air into a vehicle compartment depending on the situation of contamination of air outside the vehicle compartment, for example, a system for detecting the contamination of room air by smoking or the like and controlling an air cleaner, and the like.
As is disclosed in Japanese Unexamined Patent Publication No. Hei 5-157714, for example, there has been known a gas detecting device using a second differential value obtained by differentiating the output signal of a gas sensor element or A/D converting an analog differential value and then digitally differentiating the converted value in order to enhance the sensitivity of the change in a gas concentration to quickly take a countermeasure against the change in the concentration and to enhance S/N for a noise such as a wind. In International Patent Publication No. 501095/1989, moreover, there has been disclosed a gas detecting device for integrating a sensor signal and comparing the integral value with the sensor signal to detect a gas.
However, the gas detecting device using the gas sensor element in which a sensor resistance value is changed by a variation in the concentration of a specific gas has such a property that the sensor resistance value also fluctuates by the influence of other environments such as a temperature, a humidity or a wind velocity in addition to the change in the concentration of the specific gas. For this reason, the gas detecting device using a differential value detects a relative change in an output signal. The output signal is greatly changed by other environments such as a temperature, a humidity or a wind velocity in addition to the change in the concentration of the specific gas. Therefore, it is impossible to decide whether the output signal is changed by the concentration of the specific gas or a disturbance such as a change in the humidity based on only the relative change in the output signal. More specifically, in the case in which the differential value or the second differential value of the output signal of the gas sensor element is used, it is possible to grasp a time that the gas concentration fluctuates (for example, the gas concentration is rapidly increased). However, it is hard to know the degree of the change in the gas concentration, the subsequent situation of the change in the gas concentration or a time that the gas concentration is reduced.
On the other hand, in the gas detecting device for detecting a gas by comparing the integral value of a sensor signal with the sensor signal itself, a change in the integral value is more delayed than a change in the concentration of a specific gas. Therefore, when the concentration of the specific gas is once started to be reduced, the integral value becomes greater than the sensor signal in some cases. For this reason, when the concentration of the specific gas is then increased again, the increase in the concentration of the specific gas cannot be detected early so that the change in the concentration of the specific gas cannot be detected properly, that is, a detection timing is delayed because the integral value is greater than the sensor signal irrespective of the start of the increase in the concentration of the specific gas (therefore, the sensor signal).
In order to detect an increase and a reduction in the concentration of the specific gas, it is expected that an increase period for the concentration should be grasped as early as possible while relieving the influence of the environment such as a temperature or a humidity, that is, an initial stage for the increase in the concentration of the specific gas should be grasped to generate a signal indicating that the concentration of the specific gas is increased. On the other hand, it is expected that a signal indicating that the concentration is reduced should be generated after the concentration is sufficiently reduced in a reduction period for the concentration. As in the conventional art using differentiation, integration or the like in which both of the increase and the reduction in the concentration are decided based on a value (a differential value or the like) calculated by the same calculating method, however, it is hard to properly grasp both the initial stage of the increase and the reduction period for the concentration to generate each signal.
The invention has been made in consideration of such problems and has an object to provide a gas detecting device capable of properly grasping increase and reduction periods for the concentration of a specific gas to generate each signal, that is, a gas detecting device capable of relieving the influence of an environment such as a temperature, a humidity or a wind velocity to detect a change in the concentration of the specific gas with high precision and an autoventilation system for a vehicle using the gas detecting device.